In FIG. 1, a structure 1 with electrodes is intended to be connected to a downstream circuit 2. Structure 1 comprises an insulating flexible support film 3. On film 3 are deposited electrodes 4 connected by conductive tracks 5 to connection pads 6. The electrodes, the pads, and the tracks connecting them are formed by deposition and etching of a conductive layer on film 3. A thin insulating layer, not shown, covers the structure, except for the electrodes and the pads. Structure 1 is a flexible structure of small thickness (from a few micrometers to a few tens of micrometers), the electrodes of which are intended to be placed to contact an organ such as a nerve or the skin to measure the electric activity of the organ or to stimulate it.
Structure 1 must be connected to circuit 2. Circuit 2 comprises pads 7, the arrangement of which corresponds to that of pads 6. Each pad 7 is connected to a conductive track 8 for the conduction of the signal provided to or by electrodes 4. To connect a pad 6 to a pad 7, a hole 10 thoroughly crossing the structure is made in each of pads 6.
In FIG. 2, downstream circuit 2 is partially shown and a single contact pad 7 is visible. Structure 1 is here shown with support film 3, a single pad 6, and an upper insulating layer 11. The thickness of circuit 2, on the order of from one to two millimeters, is significant as compared to that of structure 1, which is at most a few tens of micrometers. Structure 1 is placed on circuit 2 so that each of holes 10 is above a pad 7. The surface of each pad 7 is greater than that of a hole 10. A welding drop 12 is then deposited, by a conventional bonding technique well known in microelectronics, which fills hole 10 and overflows on pad 6. Welding drop 12 ensures the electric contact between pads 6 and 7.
This way of doing has several disadvantages.
The electrode-supporting structure must be drilled into in the middle of each pad 6. This operation is delicate, due to the fragility of the structure, and this assumes that pads 6 are large enough, which limits their number. The drilling of pads 6 is generally performed by means of a laser beam, which results in having the metal layer forming the pad partially split, so that the pad no longer exhibits a uniform surface. The drilling of pads 0.6 may also be carried out by etching, which requires an additional mask, which brings about a cost problem, and greatly increases the pad drilling time.
Further, the impedance exhibited by this type of contact is relatively high. Indeed, the surface of pad 6 enabling flowing of an electric signal between pads 6 and 7 is a substantially ring-shaped surface 13 delimited by the base of welding drop 12 and the circumference of hole 10. The surface of pad 7 enabling flowing of the electric signal is a substantially circular surface 14 equal to the surface area of hole 10. Surfaces 13 and 14 have been shown in bold lines in FIG. 2. To avoid too much decreasing the impedance of the contact between pads 6 and 7, surfaces areas 13 and 14 must be substantially equal. As a result, at most, the surface area enabling flowing of an electric current between pads 6 and 7 is equal to half the surface area of pad 6, which makes the impedance of the contact between pads 6 and 7 relatively high.
Further, welding 12 forms a brittle dome of relatively great height, typically on the order of 50 micrometers. This may hinder the circuit encapsulation.
German patent application DE 19530353 A1 generally describes a method for connecting contact areas of a flexible film to a printed circuit. In this document, the printed circuit has pads drilled with metallized holes, the metallization extending somewhat over the rear surface of the integrated circuit which does not support the pads. The portions of the film and of the circuit to be connected are opposite and a filler metal is deposited between them. Heat is provided to the rear circuit surface, while a relatively high pressure is applied on the assembly formed by the film and the circuit. The provided heat melts the filler metal, which forms a relatively thick gluing layer between the film and the circuit. The excess filler metal is absorbed by the metallized hole.
The abstract of Japanese patent JP 09312453 describes a technique similar to that described in application DE 19530353.
The techniques described in the two above documents require application of a high temperature and pressure. When such techniques are desired to be applied to connect a structure support electrodes intended to measure the electric activity of an organ or to stimulate it, several problems arise. Indeed, electrode structures are very thin and very brittle. The application of a pressure, even mild, crushes the structure and may damage it. Further, the application of a high temperature, necessary to melt the filler metal, may destroy the structure. It will be seen hereafter that the overthickness created by the welding layer may also be a disadvantage.